(A) Field of the Invention
The present invention is related to a memory management method, and more specifically, to a management method for a non-volatile memory.
(B) Description of the Related Art
In recent years the demand for storage systems is increasing due to the rapid development of modern consumer products, such as thumb drives, digital cameras, MP3 players, etc. Larger storage spaces allow for storage of more information or data. To maintain the data when the devices are off, flash memory is indispensable to these devices. However, with the continuing size reduction, flash memory faces many serious issues in yield, reliability, and endurance. To increase the yield, built-in self testing and diagnosis designs are often added. As for increasing the reliability and endurance, redundant elements (i.e. spare elements) and fault tolerant methods are often proposed. One of the cost-effective fault tolerant designs for memory is the error-correction code (ECC). By applying the ECC mechanism to memory, a certain number of errors can be corrected or detected. Due to the endurance and reliability issues of flash memory, ECC is becoming increasingly important to flash memory. One of the common error-correction codes in commercial NAND flash is the Bose-Chaudhuri-Hocquenghem (BCH) code. The BCH code performs multiple-error correction. It is widely used in flash memory. The BCH code can be defined easily by two parameters selected by the designer.
Although flash memory uses the ECC for improving reliability and endurance issues, the reliability of a flash memory nevertheless decreases with time when it is in user mode. This is because program and erase operations of flash memory are destructive. The more users store or delete data in flash memory, the more cells will be damaged. Finally, the errors will exceed the designed correction capability and the flash memory device can no longer be repaired. Under such situation, the entire flash memory module or memory block must be abandoned while most of its memory cells are still functioning. Consequently, the lifetime of the flash memory device is significantly less than the lifetime of most of its memory cells.